1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to an active-matrix liquid crystal display device and a method of displaying an image on such an active-matrix liquid crystal display device.
2. Description of the Related Art
One process of applying an electric field to the liquid crystal of a liquid crystal display device is a static driving process by which a constant voltage signal is steadily applied to each electrode. If a large amount of information is to be displayed on a liquid crystal display device according to the static driving method, then a very large number of signal lines would be required. For displaying such a large amount of information on a liquid crystal display device, a multiplex driving method is usually employed for supplying a multiplexed signal voltage to electrodes. In several versions of the multiplex driving method, active matrix method holds electric charges applied to electrodes until a next frame is supplied, so that high-quality images can be displayed.
The electric field may be applied to the liquid crystal either perpendicularly to glass substrates which sandwich the liquid crystal or parallel to the glass substrates (In-Plane Switching, or IPS, method). The latter method lends itself to being applied to large-size display monitors because it allows a wide angle of view. FIG. 1 of the accompanying drawings shows a conventional electrode structure of a unit pixel of a display pixel according to the IPS method, as disclosed in Japanese patent publication No. 63-21907. The present invention is concerned with an active-matrix liquid crystal display device in which an electric field is applied to the liquid crystal parallel to the glass substrates according to the IPS method.
The conventional electrode structure will be described below.
FIG. 1 shows in plan the conventional electrode structure, and FIG. 2 of the accompanying drawings is a cross-sectional view taken along line II--II of FIG. 1. As shown in FIG. 2, a glass substrate 101 of a thin-film transistor (TFT) as an active device supports common electrodes 103 and an interlayer insulating film 105 thereon, and pixel electrodes 104 and signal lines 102 disposed on the interlayer insulating film 105. The pixel electrodes 104 and the common electrodes 103 are positioned alternately with each other. The pixel electrodes 104 and the signal lines 102 are covered with a protective insulating film 106 which is coated with a TFT alignment film 107 that is rubbed in a direction to orient liquid crystal molecules. The glass substrate 101, the common electrodes 103, the interlayer insulating film 105, the pixel electrodes 104, the signal lines 102, the protective insulating film 106, and the TFT orientation film 107 jointly make up a TFT substrate assembly 100.
A matrix-shaped light shield film 203 is disposed on an opposed glass substrate 201, and a color layer 204 for displaying colors is disposed on the light shield film 203. A planarizing film 202 for providing a flat surface on the opposed glass substrate 201 is disposed on the light shield film 203 and the color layer 204. The planarizing film 202 is coated with an opposed-substrate orientation film 207 that is rubbed in a direction opposite to the direction in which the TFT alignment film 107 is rubbed. The opposed glass substrate 201, the light shield film 203, the color layer 204, the planarizing film 202, and the opposed-substrate alignment film 207 jointly make up an opposed substrate assembly 200.
A liquid crystal material 301 is sealed between the TFT substrate assembly 100 and the opposed substrate assembly 200. A TFT polarizer 110 whose axis of transmission lies perpendicularly to the rubbed direction is attached to a surface of the glass substrate 101 remote from the common electrodes 103. An opposed-substrate polarizer 205 whose axis of transmission lies perpendicularly to the axis of transmission of the TFT polarizer 110 is attached to a surface of the opposed glass substrate 201 remote from the light shield film 203 and the color layer 204.
The TFT substrate assembly 100, the opposed substrate assembly 200, the liquid crystal material 301, the TFT polarizer 110, and the opposed-substrate polarizer 205 jointly make up a liquid crystal display panel 300.
Operation of the liquid crystal display panel 300 will be described below.
FIG. 3 of the accompanying drawings shows a rectangular area of FIG. 2 at an enlarged scale, the view being illustrative of the manner in which the liquid crystal display panel 300 operates. In FIG. 1, a TFT 109 is switched on and off by an on/off signal from a scanning line 108 that is disposed in the same layer as the common electrodes 103. When the TFT 109 is turned on, charges from the signal lines 102 flow into the pixel electrodes 104. After the TFT 109 is turned off, the pixel electrodes 104 hold the charges and are kept at a certain potential. A constant DC voltage is applied at all times to the common electrodes 103. Due to a potential difference between the pixel electrodes 104 and the common electrodes 103, a horizontal inter-electrode electric field is developed parallel to the glass substrates. As shown in FIG. 3, liquid crystal molecules 302 are rotated, causing a change in light retardation to change the amount of light transmitted through regions where the light shield film 203, the pixel electrodes 104, the common electrodes 103, the scanning lines 108, and the TFT 109 are not present. The liquid crystal molecules 302 are rotated in the illustrated direction when the dielectric constant anisotropy of the liquid crystal material is positive and in the direction opposite to the illustrated direction when the dielectric constant anisotropy of the liquid crystal material is negative.
FIG. 4 of the accompanying drawings illustrates the basic principles of the IPS method. The IPS method will be described with respect to a liquid crystal material whose dielectric constant anisotropy is positive. The liquid crystal molecules 302 are initially oriented perpendicularly to the axis of transmission of the TFT polarizer 110. Since incident light applied to the liquid crystal display panel 300, which has been polarized by the TFT polarizer 110, is not retarded by the liquid crystal material 301, the incident light is blocked almost completely by the opposed-substrate polarizer 205. At this time, the liquid crystal display device displays a black image.
When a horizontal electric field generated between the pixel electrodes 104 and the common electrodes 103 is applied to the liquid crystal molecules 302, the liquid crystal molecules 302 are rotated. Then, incident light applied to the liquid crystal display panel 300 is retarded due to the refractive index anisotropy of the liquid crystal molecules 302, and generally becomes elliptically polarized immediately before it passes through the opposed-substrate polarizer 205. The component of the elliptically polarized light which lies parallel to the axis of transmission of the opposed-substrate polarizer 205 is emitted from the liquid crystal display panel 300. The time average of the intensity of the emitted light is visually perceived by the observer of the liquid crystal display panel 300. The pattern of the elliptically polarized light varies depending on the angle .psi. formed between the average direction in which the liquid crystal molecules 302 are directed and the direction in which the liquid crystal molecules 302 are initially oriented. A standardized transmittance T/T.sub.0 of the liquid crystal display panel 300 at this time may be approximated by the following equation (1): EQU T/T.sub.0 =sin .sup.2 (2.psi.) sin .sup.2 (.DELTA.n.multidot.d.pi./.lambda.) (1)
where .psi. is the angle formed between the average direction in which the liquid crystal molecules 302 are directed and the direction in which the liquid crystal molecules 302 are initially oriented, .DELTA.n the refractive index anisotropy of the liquid crystal molecules 302, d the cell gap, and .lambda. the wavelength of the transmitted light.
As can be seen from the equation (1), the transmittance is minimum when .psi.=0.degree. and maximum when .psi.=45.degree..
The above liquid crystal display device suffers the following problems:
When an image displayed by the liquid crystal display device is viewed at a relatively large viewing angle .theta. (see FIG. 7 of the accompanying drawings), the displayed image becomes bluish or yellowish (color tint) due to the refractive index anisotropy of the liquid crystal molecules. As shown in FIG. 5 of the accompanying drawings, the displayed image is tinted bluish when it is viewed along the major axis of the liquid crystal molecule 302, and tinted yellowish when it is viewed along the minor axis of the liquid crystal molecule 302. FIG. 6 of the accompanying drawings shows x-y chromaticity changes caused when a medium-tone image displayed on the liquid crystal display device is viewed at the viewing angle .theta.=60.degree. and an azimuth angle .phi.=0.degree.-360.degree.. The viewing angle .theta. and the azimuth angle .phi. are defined shown in FIG. 7. The arrows represent chromaticity coordinates at the time the liquid crystal display device is viewed head-on. It will be understood that the displayed color is greatly shifted toward a bluish tint or a yellowish tint when viewed in an oblique field of view. The mechanism of the above phenomenon will be described below.
FIG. 8 of the accompanying drawings shows theoretical formulas for an effective refractive index anisotropy .DELTA.n' and an effective cell thickness d' when a liquid crystal molecule is viewed at a viewing angle .theta. along the major and minor axes of the liquid crystal molecule.
FIG. 9 of the accompanying drawings shows the dependency on the viewing angle .theta. of the effective light retardation (.DELTA.n'.multidot.d') when the liquid crystal molecule is viewed along the major and minor axes of the liquid crystal molecule, according to the theoretical formulas with typical characteristic values substituted in the parameters.
According to the equation (1), the relationship between a certain retardation .DELTA.n.sub.0 .multidot.d.sub.0 and the maximum wavelength .lambda..sub.0 of the light transmitted with the retardation is given by the following equation (2): EQU .DELTA.n.sub.0 .multidot.d.sub.0 /.lambda..sub.0 =1/2 (2)
Thus, the retardation and the maximum wavelength of the transmitted light are proportional to each other.
From FIG. 9 and the equation (2), it can be understood that the displayed image becomes bluish when viewed along the major axis of the liquid crystal molecule 302 and yellowish when viewed along the minor axis of the liquid crystal molecule 302, as shown in FIG. 5.
FIG. 10 of the accompanying drawings schematically shows orientations of liquid crystal molecules across the cell when (a) a black image, (b) a half-tone image, and (c) a white image are displayed. Because of constraining forces from the substrate surfaces, the liquid crystal molecules are directed more uniformly as the color of the displayed image changes toward black. The color tint is greater as the color changes toward black where the liquid crystal molecules are aligned. However, since the color tint does not manifest itself unless there is a certain amount of lightness in the displayed image, the color tint is most conspicuous in the middle-tone image.
One arrangement for improving the color tint is disclosed in Japanese laid-open patent publication No. 9-105908. According to the disclosed arrangement, common and pixel electrodes are disposed obliquely to each other for rotating liquid crystal molecules bidirectionally. While the disclosed proposal is highly effective to reduce the color tint, the aperture ratio is lowered because the common and pixel electrodes are disposed obliquely to each other. Furthermore, in a boundary region where the liquid crystal molecules rotate bidirectionally, a disclination due to a reverse twist of the liquid crystal material occurs and is visually recognized as an after image.